1. Field of the Invention
The present invention relates to an image forming apparatus that forms a color image by superimposing images of various colors on a transfer belt, particularly to an image forming apparatus capable of correcting the color misregistration and deviation of a transfer belt.
2. Description of the Related Art
In an image forming apparatus such as a tandem color photocopier using an electrophotographic process, an image forming section including a photosensitive drum, charging device, optical scanning device and development device is provided for each of yellow (Y), magenta (M), cyan (C) and black (K) colors. These devices are arranged along an endless intermediate transfer belt, and a color image formed by superimposing the images of Y, M, C and K colors on the rotating intermediate transfer belt is transferred from the intermediate transfer belt to transfer paper.
When images of various colors are superimposed to form a color image as described above, the image formation positions of different colors must be correctly aligned. Otherwise, color misregistration will occur and a high-quality image cannot be obtained. To solve this problem, a color registration correction method (Japanese Unexamined Patent Application Publication No. 1-142679) has been used as required, wherein a test image for color registration correction known under the name of a registration mark is formed on the intermediate transfer belt, and this image is read by an optical sensor to get the required correction value, whereby the image forming positions for image forming sections of different colors are aligned.
Another solution to the aforementioned problem is provided by the correction control method (Japanese Unexamined Patent Application Publication No. 9-48533) wherein stable traveling of the intermediate transfer belt is achieved by preventing meandering and deviation.
The correction control technique for the deviation of the intermediate transfer belt is normally carried out throughout the process of image formation in order to avoid color misregistration resulting from the meandering or deviation of the intermediate transfer belt. However, if the deviation of the intermediate transfer belt is corrected during the correction of the color registration, the amount of color misregistration measured according to the registration mark will be equal to the sum of the color misregistration caused by the misalignment of the image forming position due to the image forming section and the color misregistration caused by the traveling of the intermediate transfer belt due to deviation. Thus, if color registration is executed according to the measured color misregistration, an error results from the amount of corrections having been made.
In the meantime, if the correction control for the deviation of the intermediate transfer belt is suspended during the correction of color registration, and the intermediate transfer belt is misaligned during the correction of color registration, the amount of color registration correction will be inappropriate. Not only that, this will require much time and effort in returning the deviated transfer belt to the appropriate position upon completion of correcting the color registration.
The object of the present invention is to solve the aforementioned problems and to provide an image forming apparatus capable of providing adequate correction of color registration while preventing the deviation of an intermediate transfer belt.